There is something powerful about understanding that something can happen, and then seeing it happen. Today a video on BBC showed what happens when a lava flow moving gently down a snow-covered slope suddenly becomes very dangerous. The video clip shows a white plume of steam with some dark streaks in it, then suddenly there are loud wooshing noises and bangs. The rising plumes are black and brown. The camerawoman needed to run to escape. The group thankfully all made it away from the scene and down slope.
Video from BBC from March 16, 2017.
So what happened? How does a lava flow suddenly explode? It is all about the snow. The lava flow is very hot > 1,000 C and the snow is frozen, so 0 C. That big difference represents all the energy in the lava flow in the form of heat. We complain when the weather is above 37 C (98.6 F; or hotter than our inside body temperature), so you can appreciate that there is a LOT of heat energy in that lava flow. We might imagine that lava would melt all the snow away rapidly. Turns out it is a bit more complicated than that.
This video taken on Tolbachik volcano in Kamchatka, Russia in 2013 is a great example. During this eruption there was ample snow for the lava to travel on. You can see the lava touches the snow without instantly melting it. This is because of how heat is transferred through the lava. Thermal diffusivity is a way of measuring the ability of the material to transfer heat as well as how well it stores heat. Lava is good at storing heat. It takes a long time for a lava flow to cool down. You can think of different materials in your house that have various thermal diffusivities, like that aluminum pan you always burn your hand on that gets hot super fast (and cools down really fast too). There is also that cast iron pan that takes forever to get hot, and stays hot much longer than you even need it for cooking (when you think lava think of the cast iron pan).
So if lava likes to keep its heat, how do we get those explosions? The snow does melt, just not as fast as you might expect. In fact much of the snow can be turned into steam because we are well above the boiling point of water (100 C). If this steam has an easy escape it can just billow out the side of the lava flow and produce white clouds. If the steam builds up under the lava it can sometimes escape rapidly and produce that wooshing noise and fast rising clouds (like in the Tolbachik video). But what we saw on Etna had dark clouds that thew hot rock at the camera crew?!
This is a more complex process. When the snow melts, some of it hangs around as liquid water. Water is much better at transferring thermal energy than steam. If the water gets into the lava flow through a crack, or if the lava traps pieces of snow in it while traveling, it can produce an explosion flinging steam and hot rock into the air and make it dangerous for observers. So what is happening with the trapped water that is different? The space (volume) that steam requires is much greater than that of water, so there is an element of the explosion caused by the pressure build up of steam. There is also a process called 'molten fuel coolant interaction'. Since water is more efficient at heat transfer it can transfer thermal energy (heat) rapidly from the lava. This causes the lava to crack as it cools. If this happens really quickly it shatters the lava and produces a pressure wave, turning the thermal energy into mechanical energy. In order for enough heat to be transferred at a fast enough rate to lead to an explosion, a large surface area of hot lava needs to be in contact with liquid water. In fact, the more we study this we realize that the two materials need to be mingled together so that there is lots of water in contact with the lava.
Water droplet on a hot pan dancing on top of a protecting steam layer. From CoolScienceGifs.tumblr
All along this surface the water wants to turn to steam. This steam actually insulates the water from the lava, like the water droplet in the gif above. This protects the liquid water and lets it accumulate under the lava flow. If the vapor film breaks down all at once, from a sudden shift in pressure caused by the moving flow or the shaking ground, the water is put in contact with the lava all at once and cools the surrounding lava rapidly. This shock to the lava, plus expanding steam, breaks up and throws rock at high speeds (an explosion).
The famous Kilauea firehose from February 2017 shows explosions from the lava rushing into the sea, video courtesy of USGS.
If you watch these various videos you can see this full range of interactions. Gentle melting, lazy looking steam plumes, rapid steam plumes, small explosions throwing a few bits of rock, and bigger explosions that break up and throw larger volumes of the lava. I think the Etna event was getting closer to the more explosive end member from just how much material was involved in the explosion. What is really important to notice is that the explosion doesn't happen the whole time. In fact there are lots of chances for lava to interact with water and snow without exploding! There are lava flows in the ocean that predominantly make bulbous shapes called pillow lavas without exploding.
A growing pillow lava, you can see the steam enveloping the lava as it grows, insulating the lava and slowing down the cooling process. From .gify original source unknown.
This is because the circumstances that lead to the explosions (trapping water and vapor) and the right amount of surface area are less common. All this complexity, and the difficulty of studying volcanoes that might throw hot rock at you, is why the question of 'whether interactions explode or not' is one of the biggest questions in volcanology at the moment.
My own research focuses on the rocks formed by both explosive and non-explosive interactions between magma and lava with water on Earth and on Mars. I've written about how volcanoes involving lots of magma water interactions are different from your typical volcano and what sort of crazy things happen with eruptions under glaciers before on the blog, and I expect there will be more in the future (especially after some field work I'm doing in Idaho). You can find examples of magma water interactions all over the world. We've seen evidence for lava flows interacting with water before including some spectacular features called rootless cones or pseduocraters, which are craters on top of a lava flow caused by multiple explosions from trapped water in the same spot.
Rootless cones in Myvatn, Iceland, the small craters along the lake shore. These formed where the lava flow went into a lake. Picture from 2011 by Graettinger.
If you want some more technical reading about molten fuel coolant interactions in volcanic settings here are some papers to look up:
Zimanowski, B., Büttner, R. & Lorenz, V. Bull Volcanol (1997) 58: 491. doi:10.1007/s004450050157 Mattox and Mangan JVGR (1997) Littoral hydrovolcanic explosions a case study of lava-seawater interaction at Kilauea volcano. 10.1016/S0377-0273(96)00048-0
(open access) Hamilton, C.W., Fitch, E.P., Fagents, S.A. et al. Bull Volcanol (2017) 79: 11. doi:10.1007/s00445-016-1086-4
And the citations with these papers.